1. Field of the Invention
The present invention relates a display device using organic electroluminescence elements, light-emitting diodes or other similarly light-emitting elements. More particularly, the present invention relates to a display device having a driving circuit which can suppresses changes of light emission intensity of the light emitting elements.
2. Description of the Related Art
Display devices are known employing for example organic EL (electroluminescence) elements. Organic EL elements can be driven with low DC voltage. In addition, organic EL elements are light-emitting elements, so, compared with optically transparent elements such as liquid-crystal elements, they provide a wide field of view angle, a bright display surface and are of small thickness and light weight. Organic EL elements can therefore be employed as large-capacity display devices for various applications.
A technique for driving organic EL display devices is disclosed in for example Japanese Laid-open publication number 301355/1994.
The electrical characteristic of an organic EL element is disclosed in FIG. 7 of this publication. An organic EL element emits light when current flows in the forward direction between the anode and cathode. However, the light emission intensity of an organic EL element depends not merely on the current between the anode and cathode but also on the voltage between the anode and cathode. Consequently, in order to match the light emission intensity of the organic EL element accurately with the design value, it is necessary to control both the current and the voltage between the anode and cathode.
An organic EL display device comprises a large number of organic EL elements arranged in matrix fashion. With such a construction, when a large number of organic EL elements emit light simultaneously, the amount of current flowing to ground becomes very large. The cathode potential of the organic EL elements therefore rises, due to the internal resistance of the drive circuit. Consequently, the voltage between the anode and cathode of the individual organic EL elements is decreased. That is, the light emission intensity of the individual organic EL elements may be lowered due to a large number of organic EL elements emitting light simultaneously.